Virgin Birth: Female Zebra Shark Has Babies Without Mating

Sure, she used to have a mate at the Reef HQ Aquarium in Townsville, Australia. The pair even had several litters before they were separated in 2012.

But Leonie had been living apart from males for the past few years, so her keepers were surprised when she laid eggs that produced three baby sharks in April 2016. Leonie could be the first shark ever observed to make the switch from sexual to asexual reproduction.

“We thought she could be storing sperm; but when we tested the pups and the possible parent sharks using DNA fingerprinting, we found they only had cells from Leonie,” said University of Queensland biologist Christine Dudgeon, who described the case in the journal Scientific Reports Monday (Jan. 16).

Parthenogenesis occurs when embryos develop and mature without fertilization by a male’s sperm. Rather, an egg progenitor cell that usually gets absorbed by the female’s body acts as a surrogate sperm to “fertilize” her egg. This reproduction strategy is more common in plants and invertebrate organisms. However, scientists have been documenting an increasing number of vertebrate species that can have virgin births even when their species normally reproduces sexually. For example, Komodo dragons, the world’s largest lizards, have given birth by parthenogenesis. So have wild pit vipers, blacktip sharks, chickens and turkeys.

In most of these previous parthenogenesis cases, the females were from captive environments and never had any exposure to male mates during their reproductive prime, Dudgeon and her colleagues wrote. That makes Leonie one of the rare individuals known to have had babies by sexual reproduction only to switch to asexual reproduction later on. (Scientists have reported similar cases in a boa constrictor and an eagle ray.)

“Leonie adapted to her circumstances, and we believe she switched because she lost her mate,” Dudgeon said in a statement. “What we want to know now is, ‘Could this occur in the wild?’ and, if so, ‘How often does it?’ One reason why we haven’t seen it before could be because we haven’t been looking for it. It might be happening in the wild, but it’s never been recorded in this species before.”

If parthenogenesis is indeed an evolutionary adaptation to a lack of suitable mates, that could have implications for the survival of zebra sharks. The species, which is found in the western Pacific and Indian oceans, is listed as endangered on the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species.

Dudgeon plans to monitor Leonie’s pups to find out if these asexually produced sharks can have pups of their own with a male partner.

“You lose genetic diversity with generations of asexual reproduction, so we’ll be seeing if these offspring can mate sexually themselves,” Dudgeon said.

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